Delayed-setting concrete

ABSTRACT

The invention relates to a delayed-setting concrete based on blast furnace slag. The invention further relates to a method of initiating the setting of said concrete.

[0001] The present invention relates to the building and constructionsector. More precisely, the invention relates to a concrete whosesetting is initiated after it has been poured, and to a method forinitiating the setting of said concrete

[0002] The concrete currently in use in special works has a rheologywhich is not adapted to the criteria for the preparation of deepfoundations.

[0003] The major problem is the change in the rheology. In fact, theinitial set time is very often shorter than the concreting time; thisresults in the production of structures in which the following areobserved:

[0004] poor coating of the reinforcements,

[0005] inclusions of drilling sludge,

[0006] poor adhesion between the concrete and the reinforcements.

[0007] Attempts to modify these concretes with adjuvants do not easilymake it possible, if at all, to attain the desired rheology, therebylimiting the volume of concrete pours and hence the efficiency.

[0008] The invention proposes to overcome these disadvantages by meansof a concrete whose setting is initiated after it has been poured. Saidconcrete will hereafter be called “delayed-setting concrete”

[0009] Thus, according to a first feature, the invention relates to adelayed-setting concrete based on blast furnace slag.

[0010] Said concrete advantageously consists of a mixture essentiallycomprising a blast furnace slag, aggregates such as sand and/or coarseor fine gravels, and water.

[0011] The blast furnace slag generally represents from about 10% toabout 35% by weight of the concrete composition. Although the nature ofthe blast furnace slag is not particularly critical, it is preferably ofthe basic type and the weight ratio CaO/SiO₂ is preferably between about1.10 and about 1.35. As an example of slag which can be used within theframework of the present invention, there may be mentioned a materialcomprising the following main components (in percentages by weight): 33to 40% of SiO₂, 8 to 16% of Al₂O₃, 39 to 44% of CaO, 4 to 9% of MgO and0.5 to 1.5% of a source of S²⁻ ions.

[0012] The customary aggregates used to make concrete, namely sandand/or coarse or fine gravels, generally represent from about 60% toabout 80% by weight of the concrete composition, the remainder of themixture consisting of water.

[0013] The concrete according to the invention can also comprise anyinert material commonly used in this field, for example fly ash and/orcalcareous or siliceous fillers. In this case the amount of inertmaterials used is between about 5% and about 30% by weight, based on theweight of the blast furnace slag.

[0014] According to the invention, the blast furnace slag advantageouslyhas a grain size below about 200 μm, preferably below about 100 μm. Itis desirable that this grain size should not be below about 30 μm.

[0015] The concrete according to the invention does not set as such. Ittherefore enables indefinite volumes to be concreted without the risk ofdefects induced by premature initial setting.

[0016] According to the invention, the setting of the concrete isinitiated after it has been poured into the trenches.

[0017] Thus, according to another feature, the invention relates to amethod of initiating the setting of the concrete described above, whichconsists in activating the blast furnace slag.

[0018] In one embodiment of the invention, shown in FIGS. 1A to 1C, theslag is activated with the aid of an alkaline activating agent using theprinciple of ion diffusion.

[0019]FIG. 1A shows a reinforcing cage equipped with perforated tubes 10placed about one meter apart. These tubes are covered with a non-wovenfabric to prevent the laitance from clogging them. After concreting,these tubes are filled with a solution of an alkaline activating agentsuch as sodium hydroxide, potassium hydroxide or sodium or potassiumcarbonate (FIG. 1B). After a few days, the alkaline ions migrate out ofthe perforated tubes and diffuse into the concrete, activating the blastfurnace slag and initiating the setting of the concrete (FIG. 1C).

[0020] In this embodiment, it is desirable to use the alkaline agent inan amount such that the final pH of the concrete is brought to a valueof between about 12 and about 13.5, preferably to a value of about 12.7

[0021]FIGS. 2A to 2C show another embodiment of the invention, whichmakes it possible to initiate the setting of the concrete more rapidly.

[0022] As in the previous embodiment, a reinforcing cage equipped withperforated tubes 10 is used (FIG. 2A). After concreting, an activatingagent is injected under pressure (FIG. 2B); it diffuses by percolationinto the concrete mass, activates the blast furnace slag and thusinitiates the setting of the concrete (FIG. 2C)

[0023] Activating agents which can be used are lime-based compounds suchas lime itself or hydrated lime (also known as slaked lime). It is alsopossible to use a sulfate-based compound such as gypsum. A mixedcompound (lime-based compound/sulfate-based compound) is also suitable;in this case the weight ratio lime-based compound/sulfate-based compoundis generally between about 0.5 and about 1.5.

[0024] In this case it is desirable to use the activating agent in anamount of between about 2% and about 15% by weight, based on the weightof the blast furnace slag.

[0025] In a variant of the embodiments described above, it is possibleto enclose the activating agent in an envelope, especially a solubleenvelope, which degrades over time and thereby allows said activatingagent to diffuse gradually.

[0026] Polyvinyl alcohol films may be mentioned as an example ofenvelopes which can be used in the method of the invention

[0027] The invention is illustrated by the Example below, which is givenpurely by way of indication.

EXAMPLE

[0028] A concrete of the following composition is prepared: Blastfurnace slag 400 kg Sand 0/5 850 kg Fine gravel 5/25 900 kg Water about200 l

[0029] This concrete does not set; setting can be initiated by one ofthe methods described above. This concrete does not therefore requirethe use of adjuvants.

What is claimed is:
 1. A delayed-setting concrete based on blast furnaceslag.
 2. A concrete according to claim 1 which consists of a mixtureessentially comprising a blast furnace slag, aggregates and water.
 3. Aconcrete according to claim 1 or 2 in which the blast furnace slagrepresents from about 10% to about 35% by weight of the concretecomposition.
 4. A concrete according to one of claims 1 to 3 in whichthe blast furnace slag has a grain size below about 200 μm, preferablybelow about 100 μm.
 5. A method of initiating the setting of theconcrete as defined in one of claims 1 to 4, which consists inactivating the blast furnace slag.
 6. The method according to claim 5 inwhich activation is effected by means of an alkaline agent such assodium hydroxide, potassium hydroxide or sodium or potassium carbonate.7. The method according to claim 6 in which the amount of alkaline agentused is such that the final pH of the concrete is between about 12 andabout 13.5.
 8. The method according to claim 5 in which activation iseffected by means of a lime-based compound, a sulfate-based compound ora mixture of a lime-based compound and a sulfate-based compound.
 9. Themethod according to claim 8 in which activation is effected by means ofa mixture of a lime-based compound and a sulfate-based compound in aweight ratio of between about 0.5 and about 1.5.
 10. The methodaccording to claim 8 or 9 in which the amount of activating agent usedis between about 2% and about 15% by weight, based on the weight of theblast furnace slag.
 11. The method according to one of claims 5 to 10 inwhich the activating agent is enclosed in an envelope which degradesover time.